Changes In Aerosol Optical Thickness Research Articles

The Impact of COVID-19 Lockdowns on Satellite-Observed Aerosol Optical Thickness over the Surrounding Coastal Oceanic Areas of Megacities in the Coastal Zone

Nearly 40 years of aerosol optical thickness (AOT) climate data record (CDR) derived from NOAA operational satellite Advanced Very High Resolution Radiometer (AVHRR) observation over the global oceans is used to study the AOT changes due to the COVID-19 lockdown over the surrounding coastal oceanic areas of 18 megacities in the coast zone (MCCZ). The AOT difference between the annual mean AOT values of 2020 with COVID-19 lockdown and 2019 without the lockdown along with the 2020 AOT annual anomaly are used to effectively identify the AOT changes that are a result of the lockdown. We found that for most of the 18 MCCZ, the COVID-19 lockdowns implemented to contain the spread of the coronavirus resulted in a decrease between 1% and 30% in AOT due to reduced anthropogenic emissions associated with the lockdowns. However, the AOT long-term trend and other aerosol interannual variations due to favorable or unfavorable meteorological conditions may mask AOT changes due to the lockdown effect in some MCCZ. Different seasonal variations of aerosol amount in 2020 relative to 2019 due to other natural aerosol emission sources not influenced by the lockdown, such as dust storms and natural biomass burning and smoke, may also conceal a limited reduction in the annual mean AOT due to the lockdown in MCCZ with relatively loose lockdown. This study indicates that the use of long-term satellite observation is helpful for studying and monitoring the aerosol changes due to the emission reduction associated with the COVID-19 lockdown in the surrounding coastal oceanic areas of MCCZ, which will benefit the future development of the mitigation strategy for air pollution and emissions in megacities.

Impact of COVID-19 Lockdowns and Australian Bushfires on Aerosol Loading over the Downwind Oceanic Regions

Nearly 40 years of aerosol optical thickness (AOT) climate data record (CDR) from AVHRR satellite observations over the global ocean is used to study the impact of COVID-19 lockdowns and Australian bushfires on aerosol loading over downwind oceanic regions. Recent similar impact studies, by contrast, were mainly focused on the urban and suburban areas near the emission sources. AOT is used as a proxy for aerosol loading in our study. We found that AOT variations due to anthropogenic emission reduction during the lockdown period are complex and non-linear. AOT reduction was observed over China’s east coastal oceans due to emissions reduction during the stringent lockdown period in China. However, an unexpected surge of AOT occurred over the coastal oceans of India during the tight lockdown period in the pre-monsoon season. Moreover, inter-annual variations of AOT, along with a long-term decreasing trend, may conceal the lockdown-related AOT changes over the US east coastal ocean and the Mediterranean Sea due to relatively relaxed lockdown measures. By contrast, AOT increases in a monotonic way due to the emissions from severe Australian bushfires in the 2019/2020 fire season. The AOT surge is evident not only over the Australian coastal ocean but also extends to the entire southern ocean. This comparative study of two opposite extreme emission scenarios in natural conditions could be a benefit in creating effective mitigation strategies for future anthropogenic emissions and air pollution.

Variations and tendencies of cloud, aerosol and solar radiation in the past decades over East of the Asian Part of Russia based on surface and satellite data records

The results of studies of atmospheric aerosol over the territory of Russia are of great interest from the point of view of environmental and climatic problems. The article presents the results of the spatial-temporal variability of aerosol optical thickness (AOT) over the Asian Part (AP) of the Russian Federation in recent decades. The Database (DB) “Aerosol optical thickness of the atmosphere from satellite and ground-based observations” created by the authors was used. The presented trends in AOT changes (annual values, summer values) and statistical estimates of real empirical series show that there is a good agreement between the annual values of AOT for satellite and ground-based values, for monthly values the discrepancy has more value. Based on observational and reanalysis data, the interannual variability of the average annual and average summer values of the total radiation flux coming to the surface was studied. The empirical typification of interannual changes in the intra-annual course of the cloud cover values (%) over the considered time interval was carried out. The analysis was performed for the Tiksi Arctic station and for the Yakutsk, Ussuriysk, Irkutsk and Yekaterinburg stations located in the AP of the Russian Federation.

Synergy of satellite-based aerosol optical thickness analysis and trajectory statistics for determination of aerosol source regions

ABSTRACTOver 10 years of satellite-based remote sensing data with trajectory statistics are used to determine sources of air mass related to different aerosol types. Aerosol typing is based on analysis of sun-photometer data and then extended in time with the use of satellite data. Supplemental analysis of Gaussian distributions of aerosol optical thickness changes during transport, based on Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, is performed. It allows for differentiating between primary and secondary aerosol sources as well as to prove air mass source as a source of aerosol. It appears that source regions of air mass and aerosols’ sources are collocated in most cases. For western advection from Germany, the most probable aerosol sources are located between air mass source regions and the receptors. These are mostly associated with the industrial/urbanized aerosol type. Additionally, aerosols from intermittent and strong episodes of biomass burning to the east of the Polish border are found to originate from sources located closer to the receptors than backward trajectory analysis alone suggested.

Climatology Analysis of Aerosol Effect on Marine Water Cloud from Long-Term Satellite Climate Data Records

Satellite aerosol and cloud climate data records (CDRs) have been used successfully to study the aerosol indirect effect (AIE). Data from the Advanced Very High Resolution Radiometer (AVHRR) now span more than 30 years and allow these studies to be conducted from a climatology perspective. In this paper, AVHRR data are used to study the AIE on water clouds over the global oceans. Correlation analysis between aerosol optical thickness (AOT) and cloud parameters, including cloud droplet effective radius (CDER), cloud optical depth (COD), cloud water path (CWP), and cloud cover fraction (CCF), is performed. For the first time from satellite observations, the long-term trend in AIE over the global oceans is also examined. Three regimes have been identified: (1) AOT < 0.08, where CDER increases with AOT; (2) 0.08 < AOT < 0.3, where CDER generally decreases when AOT increases; and (3) AOT > 0.3, where CDER first increases with AOT and then levels off. AIE is easy to manifest in the CDER reduction in the second regime (named Regime 2), which is identified as the AIE sensitive/effective regime. The AIE manifested in the consistent changes of all four cloud variables (CDER, COD, CWP, and CCF) together is located only in limited areas and with evident seasonal variations. The long-term trend of CDER changes due to the AIE of AOT changes is detected and falls into three scenarios: Evident CDER decreasing (increasing) with significant AOT increasing (decreasing) and evident CDER decreasing with limited AOT increasing but AOT values fall in the AIE sensitive Regime 2.

Trend estimates of AERONET-observed and model-simulated AOTs between 1993 and 2013

Abstract Recently, temporal changes in Aerosol Optical Thickness (AOT) have been investigated based on model simulations, satellite and ground-based observations. Most AOT trend studies used monthly or annual arithmetic means that discard details of the generally right-skewed AOT distributions. Potentially, such results can be biased by extreme values (including outliers). This study additionally uses percentiles (i.e., the lowest 5%, 25%, 50%, 75% and 95% of the monthly cumulative distributions fitted to Aerosol Robotic Network (AERONET)-observed and ECHAM/MESSy Atmospheric Chemistry (EMAC)-model simulated AOTs) that are less affected by outliers caused by measurement error, cloud contamination and occasional extreme aerosol events. Since the limited statistical representativeness of monthly percentiles and means can lead to bias, this study adopts the number of observations as a weighting factor, which improves the statistical robustness of trend estimates. By analyzing the aerosol composition of AERONET-observed and EMAC-simulated AOTs in selected regions of interest, we distinguish the dominant aerosol types and investigate the causes of regional AOT trends. The simulated and observed trends are generally consistent with a high correlation coefficient (R = 0.89) and small bias (slope±2σ = 0.75 ± 0.19). A significant decrease in EMAC-decomposed AOTs by water-soluble compounds and black carbon is found over the USA and the EU due to environmental regulation. In particular, a clear reversal in the AERONET AOT trend percentiles is found over the USA, probably related to the AOT diurnal cycle and the frequency of wildfires. In most of the selected regions of interest, EMAC-simulated trends are mainly attributed to the significant changes of the dominant aerosols; e.g., significant decrease in sea salt and water soluble compounds over Central America, increase in dust over Northern Africa and Middle East, and decrease in black carbon and organic carbon over Australia.

Near‐cloud aerosol properties from the 1 km resolution MODIS ocean product

AbstractThis study examines aerosol properties in the vicinity of clouds by analyzing high‐resolution atmospheric correction parameters provided in the MODIS (Moderate Resolution Imaging Spectroradiometer) ocean color product. The study analyzes data from a 2 week long period of September in 10 years, covering a large area in the northeast Atlantic Ocean. The results indicate that on the one hand, the Quality Assessment (QA) flags of the ocean color product successfully eliminate cloud‐related uncertainties in ocean parameters such as chlorophyll content, but on the other hand, using the flags introduces a sampling bias in atmospheric products such as aerosol optical thickness (AOT) and Angstrom exponent. Therefore, researchers need to select QA flags by balancing the risks of increased retrieval uncertainties and sampling biases. Using an optimal set of QA flags, the results reveal substantial increases in optical thickness near clouds—on average the increase is 50% for the roughly half of pixels within 5 km from clouds and is accompanied by a roughly matching increase in particle size. Theoretical simulations show that the 50% increase in 550 nm AOT changes instantaneous direct aerosol radiative forcing by up to 8 W/m2 and that the radiative impact is significantly larger if observed near‐cloud changes are attributed to aerosol particles as opposed to undetected cloud particles. These results underline that accounting for near‐cloud areas and understanding the causes of near‐cloud particle changes are critical for accurate calculations of direct aerosol radiative forcing.

Short term variability of aerosol optical thickness at Belsk for the period 2002–2010

In this work variability of aerosol optical thickness (AOT) measured at Belsk, Poland is studied as well as modification of AOT during airmass advection towards Belsk. AOT measurements taken at Belsk and at AERONET stations located in eastern Germany, Belarus and Scandinavia are used as well as satellite measurements of AOT taken by MODIS instrument onboard Terra and Aqua satellites. Directions of airmass advection are determined by means of cluster analysis of airmass backward-trajectories.Changes of AOT at Belsk from day to day varies around zero regardless of time lag between measurements. The standard deviation of these measurements increases with increasing time lag. In case of advection from west and north direction such standard deviation is reduced. It gives good perspective for a persistent forecast of next day AOT.Analysis of AOT changes during airmass advection toward Belsk reveals two modes of AOT changes distributions. One of them with small increase of AOT and second one with larger increase of AOT, so-called loading mode. Loading mode dominates in case of advection from south direction whilst the first mode of AOT changes dominates in case of advection from other directions. Mean increase of AOT associated with the first mode is 0.034 ± 0.003. Analysis of backward-trajectories shows that aerosol loading occurs over urban/industrial regions located south and south-west of Belsk. Substantial aerosol loading is found during seasonal biomass burning episodes in Eastern Europe.

Decadal changes in aerosol optical thickness and single scattering albedo estimated from ground-based broadband radiometers: A case study in Japan

[1] A method to estimate aerosol optical thickness and single scattering albedo from broadband direct and diffuse irradiances was developed. Using irradiances simulated with and without errors, the accuracies of estimated optical thickness from 0.7 to 0.8 μm and single scattering albedo in the visible wavelength region were determined to be about 0.02 and 0.05, respectively. Resulting time variations in optical thickness and single scattering albedo by broadband radiometers agreed well with sky radiometer retrievals. Long-term variations in optical thickness and single scattering albedo from 1975 to 2008 at Tsukuba, Japan, were estimated by the method described. Optical thickness increased until the mid-1980s, then decreased until the late 1990s, and was almost constant in the 2000s. The single scattering albedo was about 0.8 until the late 1980s, gradually increased, and has remained at approximately 0.9 since the mid-1990s. The surface global irradiance under clear sky conditions calculated from estimated aerosol optical properties showed an apparent transition from dimming to brightening around the mid-1980s. The magnitude of the brightening was about 12.7 W m−2; of this, 8.3 W m−2 was due to a decrease in optical thickness, and the remaining 4.4 W m−2 was due to an increase of single scattering albedo. On the other hand, the surface global irradiance measured under cloudy conditions increased by 2.6 W m−2. The dimming and brightening by aerosols were weakened by the changes in clouds. The method described could be useful in evaluating aerosol influences on long-term changes in the surface solar radiation at many sites around the world.

Global model simulations of the impact of ocean-going ships on aerosols, clouds, and the radiation budget

Abstract. International shipping contributes significantly to the fuel consumption of all transport related activities. Specific emissions of pollutants such as sulfur dioxide (SO2) per kg of fuel emitted are higher than for road transport or aviation. Besides gaseous pollutants, ships also emit various types of particulate matter. The aerosol impacts the Earth's radiation budget directly by scattering and absorbing the solar and thermal radiation and indirectly by changing cloud properties. Here we use ECHAM5/MESSy1-MADE, a global climate model with detailed aerosol and cloud microphysics to study the climate impacts of international shipping. The simulations show that emissions from ships significantly increase the cloud droplet number concentration of low marine water clouds by up to 5% to 30% depending on the ship emission inventory and the geographic region. Whereas the cloud liquid water content remains nearly unchanged in these simulations, effective radii of cloud droplets decrease, leading to cloud optical thickness increase of up to 5–10%. The sensitivity of the results is estimated by using three different emission inventories for present-day conditions. The sensitivity analysis reveals that shipping contributes to 2.3% to 3.6% of the total sulfate burden and 0.4% to 1.4% to the total black carbon burden in the year 2000 on the global mean. In addition to changes in aerosol chemical composition, shipping increases the aerosol number concentration, e.g. up to 25% in the size range of the accumulation mode (typically >0.1 μm) over the Atlantic. The total aerosol optical thickness over the Indian Ocean, the Gulf of Mexico and the Northeastern Pacific increases by up to 8–10% depending on the emission inventory. Changes in aerosol optical thickness caused by shipping induced modification of aerosol particle number concentration and chemical composition lead to a change in the shortwave radiation budget at the top of the atmosphere (ToA) under clear-sky condition of about −0.014 W/m² to −0.038 W/m² for a global annual average. The corresponding all-sky direct aerosol forcing ranges between −0.011 W/m² and −0.013 W/m². The indirect aerosol effect of ships on climate is found to be far larger than previously estimated. An indirect radiative effect of −0.19 W/m² to −0.60 W/m² (a change in the atmospheric shortwave radiative flux at ToA) is calculated here, contributing 17% to 39% of the total indirect effect of anthropogenic aerosols. This contribution is high because ship emissions are released in regions with frequent low marine clouds in an otherwise clean environment. In addition, the potential impact of particulate matter on the radiation budget is larger over the dark ocean surface than over polluted regions over land.

Multi year satellite remote sensing of particulate matter air quality over Sydney, Australia

Particulate matter (PM) air‐quality information is usually derived from ground‐based instruments. These measurements, while valuable, are not well suited to provide air‐quality information over large spatial scales. In this study, using 4 years of satellite aerosol optical thickness (AOT) at 0.55 µm derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board NASA's Terra and Aqua satellites, we present a multi‐year air analysis of PM air quality over Sydney, Australia. We then compare the satellite data with PM2.5 mass concentration measurements from six ground‐based stations in the area. Our results indicate significant diurnal variations and an overall increase in PM2.5 during Southern Hemisphere spring and summer seasons due to bush fires. The air quality in Sydney, Australia is good throughout the year except during major bushfires when PM2.5 mass loading can increase from normal (<20 µg m−3) to unhealthy conditions (>70 µg m−3). The satellite data also show corresponding AOT changes from less than 0.1 to greater than 1.0 during bushfire events. We conclude that satellite data are an excellent tool for studying PM air quality over large areas, especially when ground measurements are not available. While this is the first multi‐year combined satellite and ground‐based air quality analysis over Sydney, ancillary information from lidars, sun photometers, and size‐resolved chemistry measurements will further enhance our capability to monitor and forecast air quality in and around Sydney.

Aerosol Optical Thickness over the Oceans Derived from GMS-5 during Spring 2002 and 2003

Daily and monthly aerosol optical thickness (AOT) over the ocean, derived from the Geostationary Meteorological Satellite (GMS-5) visible data, were compared with those determined from the sun photometer measurements in March and Apri1 2002. Time variations in the daily AOT derived from the GMS-5 showed good agreements with those from the sun photometer measurements. Correlation coefficient between the GMS-5 and the sun photometer derived daily AOTs was 0.85 for 61 samples taken at three sun photometer observation sites; Ryori, Yonagunijima, and Minamitorishima in Japan. The difference of monthly AOT, the GMS-5 minus the sun photometer, in March and April 2002 were −0.03 and 0.03 at Ryori, −0.01 and 0.18 at Yonagunijima, and 0.01 and 0.07 at Minamitorishima, respectively.Monthly regional AOT, defined as the median of the daily AOTs over 30° latitude × 30° longitude region, was calculated in latitude from 60°N to 60°S and in longitude from 80°E to 160°W for March and Apri1 of 2002 and 2003. In the southern hemisphere, monthly regional AOT was less than 0.1; about 0.1∼0.2 in 0°N∼30°N; larger than 0.2 in 30°N∼60°N with a maximum of 0.4∼0.5 in the western region near the Asian continent. AOTs in 2003 were generally smaller than those in 2002 in six regions between 110°E and 160°W of the northern hemisphere, where aerosol loading is most likely affected by the Asian dust. The numbers of regions, in which the AOT increased/decreased/equivalent from 2002 to 2003, were 3/8/1 in March and April, with the AOT changes from 2002 to 2003 to be as large as −0.04∼0.03.

Estimation of diurnal shortwave dust aerosol radiative forcing during PRIDE

Using measured and derived aerosol properties from the Puerto Rico Dust Experiment (PRIDE), a four‐stream broadband radiative transfer model is used to calculate the downward shortwave irradiance (DSWI) at the surface and the shortwave irradiance at the top of atmosphere (TOA). The results of the calculated DSWI are compared against pyranometer measurements from the Surface Measurements For Atmospheric Radiative Transfer (SMART) instrument suite at Roosevelt Road (18.20°N, 65.60°W). Using aerosol optical thickness retrievals from half‐hourly geostationary satellite data (GOES 8 imager), the diurnal short wave aerosol forcing (SWARF) of dust aerosols both at the surface and TOA are calculated for the entire study area (14°N ∼ 26°N, 61°W ∼ 73°W). For selected days, the Clouds and the Earth Radiant Energy System (CERES) TOA shortwave irradiance values from Terra are compared with radiative transfer calculations. Wang et al. [2003] show that the satellite derived aerosol optical thickness is in excellent agreement with Aerosol Robotic Network (AERONET) values. Results of this study show that the calculated direct, diffuse and total DSWI are in excellent agreement with the corresponding SMART values with biases of 1.8%, −3.3% and 0.5% respectively, indicating that dust aerosols are well characterized in the radiative transfer model. This is well within the measured uncertainties (1.3%) and the model uncertainties (5%). The monthly mean value and standard deviation of aerosol optical thickness at 670 nm (AOT670) during PRIDE are 0.26 ± 0.13, and the corresponding monthly mean daytime SWARF values are −12.34 ± 9.62 W m−2 at TOA and −18.13 ± 15.81 W m−2 at the surface, respectively. Our results also show that if diurnal changes in aerosol optical thickness are not considered, it leads to uncertainties in SWARF of 4 W m−2 at the surface and 2 W m−2 at the TOA. The CERES TOA short wave irradiance underestimates calculated values by about 10 W m−2 mainly due problems in misclassification of aerosols and lack of aerosol angular dependence models (ADMs) in the current CERES algorithms. This study is among the first to demonstrate the potential of the GOES 8 imagers in retrieving aerosol optical thickness and estimating the daytime diurnal SWARF of dust, both at the TOA and surface, in low to moderate dust loading regions over the oceans.